Process for resinifying ligno-cellulosic materials with urea-formaldehyde



atented June 8, 1948 PROCESS FOR RESINIFYING LIGNO-CELLU- LOSICMATERIALS WITH UREA-FORM- ALDEHYDE William Karl Loughborough, Madison,Wis., as-

ates of America as represented by the Secretary of Agriculture NoDrawing. Application October 15, 1942, Se-

signor to United St rial No. 462,191,

tion Serial No. 383,798,

which is a division of applica- March 17, 1941. Di-

vided and this application August 21, 1944, Se-

rial No. 550,481

2 Claims.

(Granted under the act of March 3, 1883, as

amended This application is made under the act of March 3, 1883, asamended by the act of April 30, 1928, and the invention herein describedand claimed, if patented, may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment to me of any royalty thereon.

This application is a division of my copending application for patent,Serial No. 462,191, filed October 15, 1942, which is in turn a divisionof my application for patent, Serial No. 383,798, filed March 1'7, 1941,upon which Patent No. 2,313,953 issued on March 16, 1943.

My invention relates to processes for plasticizing and resinifying woodand other ligno-cellulosic materials, and to products made from saidmaterials.

For brevity and clarity whenever the word wood is used in thisspecification and the claims, it is understood that I include in itsdefinition a ligno-cellulosic complex. It is further understood thatsuch ligno-cellulosic materials may include the natural constituents oftrees and plants in varying proportions. larly trees and plants of allkinds.

A principal object of my invention is to produce solid compressed Woodof great density and strength.

A further object of my invention is to produce molded articles fromcomminuted wood.

A further object of my invention is to produce plywood and laminatedcompressed wood of great density and strength.

A further object of my invention is to produce wood which can be readilyformed when hot and wet.

A further object of my invention is to produce wood which can be readilyformed when hot and dry.

A further object of my invention is to produce a material which isthermosetting, which has very low hygroscopic properties; and which haspermanence of size and shape.

Other objects of my invention include the production of molded articlesfrom solid wood, from laminated wood, and from plywood.

. My invention, in its various modifications, is based upon the factsthat aqueous solutions of urea and thiourea, separately or in mixture,have I have in mind particu- April 30, 1928; 370 O. G. 757) a markedsoftening effect upon wood, and that urea and/or thiourea can becombined with various chemicals, such as the aldehydes, to formthermo-setting resins.

I am aware that urea and other chemicals have been used in the chemicalseasoning of wood. However, the purposes, methods employed, and resultssecured are all entirely diflerent from my invention. In chemicalseasoning it is essential that only the surface layers of the wood to bedried be impregnated with the chemical, whereas in my invention theentire volume of the wood should be impregnated. In chemical seasoning,the vapor-pressure characteristics of the aqueous solutions of thechemical and the anti-shrink pr'perties of the dry chemical within thewood are of paramount importance; whereas, in my invention the abilityof the aqueous solution of the chemicals to soften the wood and theability of said solution to condense and polymerize into athermo-setting resin are most essential properties. In chemicalseasoning it is desired to maintain the green size and form of the woodinsofar as possible; whereas in my invention an important object is tomodify the original form.

I have pointed out in Patent No. 2,298,017 that urea, NHzCONHz, andthiourea, NH2CSNH2, separately or in combination, have a markedsoftening effect upon wood at elevated temperatures, and that thiseffect may be applied to produce new and useful results. The discoveredeffects vary widely in intensity among the various ligneous andcellulosic materials of differing origins; therefore, successfulapplication of my invention will depend in large measure upon the properselection of materials, and successful utilization of any given materialwill likewise depend upon the proper selection of a suitable treating orconversion method. In general, the eiiects are most intense in the caseof ligneous materials.

I have discovered that wood which has been treated and softened bysoaking in an aqueous solution of urea and/or thiourea can be resinifiedand given superior new properties by soaking it,-

a water solution of aldepreferably before drying, in furfuryl alcohol(Cd-l'sOCI-IaOH) or of an hyde', in the presence of a catalyst.

I have also discovered that it is possible to secure a preliminarysoftening and a final resinification of the wood by the use of a singleaqueous treating solution consisting of urea and/or to a minimum of 1 to10. If maximum water resistance be desired this ratio should not belarger than 1 to 1.5.

A solution composed only of urea and/or thiourea and an aldehyde in thepresence of a suitable catalyst, but without a buffer, would beunsatisfactory for the purpose intended because it would startcondensing and polymerizing too soon. Therefore, for most of thepurposes contemplated in my invention a catalyst and a buffer must beadded. Sometimes it is expedient to use an acid catalyst and sometimesan alkaline catalyst. There is an advantage in using sodium hydroxideand enough acetic acid to form sodium acetate with some free acetic acidbecause sodium acetate will buffer the dilute organic acid solutions atpH values between 5.5 and 6.0. To avoid using an excessive amount ofhydroxide and acid, an additional buffer is required. The most efficientbufier I have discovered is borax. Some acid salts like sodium acidphosphate seem to act as a butler as well as a catalyst. In this casethe sodium acid phospate in the presence of weak wood acids holds the pHto about 8.

For use with molding powders, I find that buffers are not alwaysnecessary since the time required for the entire process iscomparatively brief.

Five typical solution formulae are here presented:

Urea grams 790 Formaldehyde (37% solution) do 714- Water do.. 485 Sodiumhydroxide do 11 Borax "do-.." 79 Glacial acetic acid cc 18 Urea grams790 Formaldehyde (37% solution) do 714 Water do 485 Sodium hydroxide do11 Borax do 79 Glacial acetic acid cc 18 Ammonia (19% solution) cc 250HI Grams Urea 790 Furfural 1,279 Water 8,000 Sodium hydroxide 240 Borax800 IV Grams Urea 790 Formaldehyde (37% solution) 714 Water 485 Sodiumacid phosphate 20 V Grams Urea 790 Water 790 Furfuryl al o 1,185Phosphoric acid (85%) 47 I have discovered and demonstrated that, tosecure best results, the solution formula must be'modifled and adjustedto suit various species and various conditions of the process.- Thus.woods like oak, containing comparatively large quantities of acid,require sparing use of acid in the solution. Further, if darkening ofthe wood be objectionable, ammonia may not be used with oak.

The buffered urea-aldehyde solution above defined and described hasoutstanding advantages over other unpolymerized or partially polymerizedresins. It will remain unpolymerized, at room temperatures, for weeks.It will also remain unpolymerized for days at temperatures up to 220 F.It will not completely polymerize at temperatures much below 300 F.These properties make possible the successful manipulation of the woodthrough the various soaking, drying, forming and compressing operationsherein described and claimed.

Through proper selection'of the chemicals I have been able to produce abuffered urea-aldehyde solution which permits satisfactory performanceof an operating schedule involving the following steps: (a) preliminarysoaking of the wood in said solution, the soaking period, everythingelse being the same, is relatively short when green wood is used andmuch longer when dry wood is used; (b) heating and softening of thesoaked wood; (c) deformation, molding, and/or compression of the heatedsoaked'wood; and (d) final condensation and polymerizing of thechemicals in said wood and drying of the finished product.

I have also been able to secure satisfactory performance of an operatingschedule involving the following steps: (a) preliminary soaking of thewood in a buflered urea-aldehyde solution for various periods, dependingupon size, species, and moisture content of the material; (b) drying ofthe soaked impregnated wood; (6) deformation, molding, and/orcompression of the dry impregnated wood at elevated temperature; and (d)fin-a1 condensation and polymerization of the chemicals within saidwood.

The preferred operating schedule, when using two separate chemicalsolutions, involves the following steps: (a) preliminary soaking of thewood in an aqueous solution of urea and/or thiourea; (b) heating thesoaked wood; (0) deformation, molding, and/or compression of the hotsoaked wood; (d) soaking said wood in an aqueous solution of an aldehydeplus a suitable catalyst; and (e) final drying of the finished product.

It is not possible, in the present state of the art, to state withcomplete certainty exactly what chemical reactions take place and whatphysical phenomena manifest themselves in the practice of my inventionbut it appears now that most of the results secured support thehypothesis'that the principal effective action during the preliminarysoaking and heating periods is one of softening. However, it isprobable, especially in the case of certain species, such as oak forinstance, which contain unusually large percentages of materials whichare neither-'cellulosic nor ligneous, that a certain amount ofcondensation and resinification does take place between the wood and thechemicals in the solution.

During the final resinifying steps in the various modifications of myprocess, the urea and/or thiourea condenses with the aldehyde andpolymerizes to form a urea-aldehyde resin. In my process, the variouschemicals in the solutions enter into the fine microscopic structure ofthe wood cells, with the end result that the resins which are formedcompletely permeate the wood structure and do not simply form asuperficial coating as in various other processes in the prior art. Thiscomplete permeation practically eliminates swelling and shrinkage withchanges in the humidity of the ambient atmosphere.

In previous experimental work I discovered that green wood may bereadily impregnated with urea and/or thiourea by immersion in an aqueous solution of said chemicals. The action is principally one ofdiffusion and it goes forward at atmospheric temperature and pressure.It may be hastened by the use of elevated temperature, but increasedpressure has only a very small accelerating effect. Dry wood can also beimpregnated in this manner, but the process is much slower than withgreen wood. I now find that wood may be impregnated with a bufferedurea-aldehyde solution quite as easily as with urea and/or thioureaalone. Other methods of impregnation may also be used in the practice ofmy invention.

It is recognized that certain properties of the finished products, suchas hygroscopicity, fireresistance, decay-resistance, resistance toinsect attack, and weather-resistance, may be modified by the additionof various well-known chemicals-principally water-soluble chemicals-tothe aqueous treating solutions. My invention contemplates the use ofsuch chemicals added to the buffered urea-aldehyde solution.

If wood be impregnated with buffeerd ureaaldehyde solution and thenheated, it becomes very soft and flexible, so that it may be easilybent, twisted, molded, compressed, or otherwise formed or deformed.

Subsequent drying and final heating to a temperature higher than that ofthe preliminary heating remove excess moisture and cause finalpolymerization of the resin-forming chemicals.

Likewise, wood impregnated in accordance with my invention and thendried and heated becomes quite soft and flexible, so that it may beformed or deformed, but to a lesser degree than when hot and wet.

The buffered urea-aldehyde has an advantage over other thermo-settingresin-forming chemical combinations in that it remains thermoplastic fora long time in the range of temperatures normally employed in kilndrying; and it does not become thoroughly cured until exposed to atemperature of about 300 F;

Final heating to a temperature higher than that of the preliminaryheating causes final polymerization to take place.

I have discovered that wood which has been impregnated with a bufferedurea-aldehyde solution possesses such a high degree of adhesivenessafter proper polymerization that, under suitable conditions, sheets canbe laminated and comminuted material molded without the addition of anyadhesive.-

My invention contemplates a method of treating, bending or otherwiseforming, and resinifying wood which consists essentially of thefollowing steps: impregnate the wood with a buffered urea-aldehydesolution; heat said wood to a temperature of about 212 F. to 220 F.;bend or otherwise form said wood; dry said wood; heat said wood to atemprature of about 300 F.

My invention also contemplates a method of treating and resinifying woodwhich consists esr 6 sentially of the following steps: impregnate saidwood with a buffered urea-aldehyde solution; dry said wood; heat saidwood to a temperature of about 300 F.

My invention also contemplates a method of treating, bending orotherwise forming, and resinifying wood which consists essentially ofthe following steps: impregnate the wood with a buffered urea-aldehydesolution; dry said wood; heat said wood to a temperature of about 212 F.to 220 F.; bend or otherwise form said wood; heat said wood to atemperature of about 300 1''.

My invention also contemplates a method of treating, densifying, andresinifying wood which consists essentially of the following steps:impregnate said wood with a buffered urea-aldehyde solution; compresssaid wood in a suitable press; dry said compressed wood under pressure;heat said wood under pressure to a temperature of about 300 F.; releasethe pressure after polymerization has been completed.

My invention also contemplates a method of treating, molding, andresinifying comminuted wood which consists essentially of the followingsteps: impregnate said wood with a buffered ureaaldehyde solution; drysaid wood at room temperature; place said wood into a suitable mold andapply pressure in a suitable press; heat said wood under pressure to atemperature of about 300 F.; release the pressure after polymerizationhas been completed.

My invention also contemplates a method of treating, laminating,compressing and/or molding, and resinifying sheets of wood whichconsists essentiallyof the following steps: impreg nate said sheets ofwood with a buffered ureaaldehyde solution; dry said sheets at roomtemperature; lay up the desired number of sheets one on top of anotherwith the grain of adjacent plies oriented in the desired manner, andplace the stack of sheets in a suitable press or mold; apply enoughpressure to said stack to secure the desired degree of compressionand/or molding, and simultaneously heat said stack under pressure to atemperature of about 300 F.; release the pressure after polymerizationhas been completed.

Specific illustrations of how my present invention may be practicedfollow:

To produce a boat rib:

Soak a stick of green white oak heartwood, one inch square and 30 incheslong, for a period of 10 days, at room temperature, in a bufferedureaaldehyde solution prepared as follows:

Mix together and heat to F.

Water grams 485 Borax do '19 Urea do 'l90 Sodium hydroxide do 11 Glacialacetic acid cc-.. 11

Cool to room temperature. When cool add 714 grams of 37 percentformaldehyde solution.

After the soaking period, heat the stick to a temperature of from 212 F.to 220 F. This may be done conveniently by immersing said stick in abath of boiling urea solution. Maintain said stick at this temperaturefor half an hour.

Remove said stick from the bath, immediately bend said stick over apreviously prepared form, and clamp said stick in place on the form. Inperforming this operation, practices customary in ordinary steam bendingmay be followed.

Allow the bent rib to dry on the form. Heat said bent rib, while stillon the form, to a temperature of about 300 F. and maintain saidtemperature for an hour. This completes the process and said bentandresinified rib may be removed from the form and immediately used as soonas it is cool'enough to handle.

To produce a light boat rib:

Soak a stick of green white oak heartwood inch by /4 inch in,crosssection and 30 inches long, for a period of five days, at roomtemperature, in a buffered urea-aldehyde solution. After the soakingperiod, dry said stick at a temperature of about 140 F. Then heat saidstick to a temperature of about 212 F. Then bend said stick, while hot,over a previously prepared form and clamp said stick in place on theform. In

performing this operation, practices customary in ordinary steam bendingmay be followed. Then heat the bent rib, while still on the form, to atemperature of about 300 F. and maintain said temperature for 40minutes. This completes the process and said bent and resinified rib maybe removed from the form and immediately used as soon as it is coolenough to handle.

To resinify a bearing block:

Soak a board of green white oak heartwood 12 inches square and one inchthick, for a period of days, at room temperature, in an aqueous solutionprepared as above specified.

' Remove said board from said solution and allow said board -to dry. 1

After said board has dried to a moisture content of about 10 percent,heat said board to a temperature of about 300 F. and maintain saidtemperature for an hour. This'completes the process and the resinifledboard may be cooled and used immediately.

To resinify and compress a die block:

Soak a board of green white oak heartwood 12 inches square and 2 inchesthick, for a period of 20 days, at room temperature, in an aqueoussolution as above specified.

Remove said board from said solution and heat said board to atemperature of from 212 F. 'to 220 F'. by immersing said board in a bathof boiling urea solution. Maintain said board at said temperature for anhour.

Remove said board from said urea solution, place said board in asuitable press and apply sufllcient pressure to reduce the thickness ofsaid board to 1 inches.

Dry said board under pressure.- A

When dry, raise the temperature of said board to about 300 F. andmaintain said temperature for one hour; I

Release the pressure and remove said board.

To mold an ash tray:

Soak a suitable quantity of fine green oak sawdust for a period of 15minutes, at room temperature, in an aqueous solution prepared as abovespecified.

Remove said sawdust from said solution and dry said sawdust at roomtemperature.

surface area of said stack.

-When dry fill the ash tray --mold withsaid sawdust, place said filledmold in a hot plate press and apply a pressure of 1000 pounds per squareinch of mold area tosaid mold.

Raise the temperature of said mold to 300 F. and maintain said pressureand said temperature for a period of 30 minutes.

Release said pressure, open said mold and remove said molded ash tray.

To make a compressed plywood plate: Soak 13 pieces of %-inch green whiteoak veneer 12 inches square for a period of 5 hours, at roomtemperature, in an aqueous solution prepared as above specified.

Remove said'veneer from said solution and dry at room temperature.

Arrange said dry veneer in a stack, with the grain of adjacent plies atright angles.

Place said stack in a hot plate press and apply a pressure of 2000.pounds per square inch of Raise the temperature of said stack to 300 F.and maintain said temperature for a period of one hour.

Open said press and remove said plate.

Having thus described my invention, I claim:

1. A process for producing molded articles of resinifled densifled woodcomprising soaking comminuted wood in a substantially unreacted aqueousformaldehyde urea solution containing an excess of urea, on an atomicweight basis, the so lution containing suitable buffers to maintain thepH at about 8; removing the powder from the solution; drying the powder;and heating the dry powder under pressure at a temperature-of about 300F. until substantially complete resinification is efiected.

2. A process for making a molding powder comprising soaking comminutedwood in a substantially unreacted aqueous formaldehyde urea solutioncontaining an excess of urea on an atomic weight basis, the solutioncontaining suitable buffers to maintain the pH at about 8, then removingthe soaked wood and drying it.

WILLIAM KARL LOUGHBOROUGH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

